The present invention relates to breathing masks. More specifically, the present invention relates to an electro-pneumatic system for controlling the air pressure within a breathing mask.
Demand type breathing apparatus has applications in such areas as medicine, scuba diving and high altitude flight. In the area of high altitude flight, the pilots of modern high-performance aircraft must be equipped with oxygen breathing systems equally capable of high performance. These systems must supply oxygen to the pilot over a broad range of operating conditions, while maintaining rapid response to the pilot's breathing demands. Moreover, a breathing system is required that maintains a positive pressure in the mask, relative to external pressure, so no toxic materials that may be present in the external environment are drawn into the mask. The conventional oxygen regulator which is currently employed to perform these functions is a product of technology which dates back to World War II. This technology will not be able to meet the future needs for ever-greater regulator performance and integrating with overall pilot life-support systems. These oxygen regulators rely on mechanical elements, such as lever arms and springs. In such regulators, some force must be applied to a closed valve in order for it to be opened and for air to be supplied. In a conventional regulator of this type, the only available source of this force is the breathing demand of the regulator user. By inhaling, the user creates a pressure differential across a diaphragm or other sensing member, resulting in the operating force. Generally, this force is lesser in magnitude than that normally required to open the valve; therefore, some device which multiplies the available force, such as a lever arrangement, is commonly used. However, because of the law of mechanical advantage, such devices will reduce the magnitude of the possible valve opening, to the same degree which they increase the available force. Since the maximum flow capability of the regulator is directly dependent upon the valve opening, this inherently limits the performance for a given diaphragm size. One can, of course, increase the diaphragm size, but in applications where space is of the essence, this is not practical. Alternate mechanical approaches have been developed, for example the balanced valve or the pilot valve, but each of these has other problems such as oscillations in the system, or the need for two steps to open the valve, respectively. Additionally, inherent in these types of mechanical systems is a lag in the response of the valve to open upon demand.
Electro-pneumatic systems do not suffer from any of the problems present in mechanical systems. Any forces necessary to open a valve are achieved through electromagnetic means, with basically a large force being achieved with, for instance a large current, in a given space. There have been attempts to produce an adequate electro-pneumatic breathing system. But, a severe problem present in them is the air supplied to the face mask is not continuous due to the servo valve not providing a linear response to a central signal which corresponds to the air pressure in the mask being greater than or less than a minimum desired pressure as a function of time. For example, United Kingdom Patent Application No. GB 2154887 (September 1985) discloses a breathing apparatus that is servo-controlled. Therein, a three-way valve comprises a circular valve member surrounded by an O-ring. The O-ring has three ports, the first port being 90.degree. apart from the second, the second being 90.degree. apart from the third, and the third being 180.degree. apart from the first. The valve member is solid except for a channel with a 90.degree. bend at the center running therethrough. In operation, the valve member is turned to a desired position by an actuator. If the pressure in the mask is too little (user inhaling), the valve rotates so the channel therein connects port 1 (connected to an air supply) and port 2 (connected to the mask) to allow air to enter the mask and raise the pressure. If the pressure in the mask is too great (user exhaling) then the valve member rotates to connect port 2 and port 3 (connected to an exhaust channel) to allow excess air to escape from the mask and reduce the pressure. The actuator operates according to a control unit that receives a signal from a pressure transducer in the mask. When the pressure is below a predetermined level, ports 1 and 2 are connected. When the pressure is above a predetermined level, ports 2 and 3 are connected. The problem with this device though, is either air is supplied or is not supplied, but there is no ability to provide air in proportion to the demand of a user.
Additional approaches have attempted to take into account all the components, i.e., g-forces, altitude, in calculating the pressure supplied, but do not deal with an electro-pneumatic system controlling the actual breathing supply. See, "Altitude and Acceleration Protection System for High Performance Aircraft" by A. Gupta and M. McGrady, Boeing Military Airplane Company, under USAF Contract F33615083-C-0651.